1. Field of the Invention
This invention relates to fuel cells.
2. Description of the Related Art
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. As shown in FIG. 1, a typical fuel cell 10 consists of a fuel electrode (anode) 12 and an oxidant electrode (cathode) 14, separated by an ion-conducting electrolyte 16. The electrodes are connected electrically to a load (such as an electronic circuit) 18 by an external circuit conductor. In the circuit conductor, electric current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H.sup.+) in acid electrolytes, or the hydroxyl ion (OH.sup.-) in alkaline electrolytes. In theory, any substance capable of chemical oxidation that can be supplied continuously (as a gas or fluid) can be oxidized galvanically as the fuel 22 at the anode 12 of a fuel cell. Similarly, the oxidant 24 can be any material that can be reduced at a sufficient rate. For specialized systems, both reactants might be liquids, such as hydrazine for the fuel and hydrogen peroxide or nitric acid for the oxidant. Gaseous hydrogen has become the fuel of choice for most applications, because of its high reactivity in the presence of suitable catalysts and because of its high energy density when stored as a cryogenic liquid, such as for use in space. Similarly, at the fuel cell cathode 14 the most common oxidant is gaseous oxygen, which is readily and economically available from the air for fuel cells used in terrestrial applications. When gaseous hydrogen and oxygen are used as fuel and oxidant, the electrodes are porous to permit the gas-electrolyte junction to be as great as possible. The electrodes must be electronic conductors, and possess the appropriate reactivity to give significant reaction rates. The most common fuel cells are of the hydrogen-oxygen variety that employ an acid electrolyte. At the anode 12, incoming hydrogen gas 22 ionizes to produce hydrogen ions and electrons. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via the metallic external circuit. At the cathode 14, oxygen gas 24 reacts with the hydrogen ions migrating through the electrolyte 16 and the incoming electrons from the external circuit to produce water as a byproduct. Depending on the operating temperature of the cell, the byproduct water may enter the electrolyte, thereby diluting it and increasing its volume, or be extracted through the cathode as vapor. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions; in the present case, the combination of hydrogen with oxygen to produce water, with part of the free energy of reaction released directly as electrical energy. The difference between this available free energy and the heat of reaction is produced as heat at the temperature of the fuel cell. It can be seen that as long as hydrogen and oxygen are fed to the fuel cell, the flow of electric current will be sustained by electronic flow in the external circuit and ionic flow in the electrolyte.
In practice, a number of fuel cells are normally stacked or `ganged` together to form a fuel cell assembly. These traditional types of fuel cells use extremely complex flat stack arrangements consisting of a membrane, gaskets, channels, electrodes and current collectors that are difficult and expensive to fabricate and assemble, and are highly subject to catastrophic failure of the entire system if a leak develops. As can be easily appreciated, the cost of fabricating and assembling fuel cells is significant, due to the materials and labor involved. Typically, 85% of a fuel cell's cost is attributable to manufacturing costs. For example, U.S. Pat. No. 5,683,828 describes a fuel cell stack employing a complex separator assembly that is a laminated structure of seven (7) layers adhesively bonded together. These laminated platelets provide humidification to the electrode assemblies and have separate channels that are dedicated to passing cooling water through the fuel cell stack for thermal management. Thus, the complexity of U.S. Pat. No. 5,683,828 and other fuel cell structures is one of the factors preventing widespread acceptance of fuel cell technology. An improved style of fuel cell that is less complex and less prone to failure would be a significant addition to the field.